Why do birds land on you?

Except in certain circumstances, birds do not 'poo' on you consciously. A bird, when it feels the need, will expel it's waste, even when flying. If you just happen to be underneath when that happens--well, you know the rest.

In some circumstances, certain birds employ a method of driving off predators, whereby they will actively 'dive bomb' the offending target (that could be you), releasing the foul smelling mixture.